H-reflex Modulation During Different Head Movements

Abstract

Head movement stimulates the vestibulospinal tract, which contributes to the balance system. Head movement increased our postural sway amplitude. The amplitude of the soleus spinal stretch reflex was decreased by contralateral side head movement. However, it has yet to be investigated if presynaptic inhibition to the soleus Ia afferents is modulated by the head movement. PURPOSE: To examine the modulation of the soleus H-reflex and presynaptic inhibition (PI) to the soleus Ia afferents during standing with head movement. METHODS: Seven young active subjects (mean: 23.3 yrs) were examined. The H-wave and M-responses of the right soleus were determined for each subject during a static standing position. Each subject turned the head up and contralaterally or ipsilaterally with or without hands on table for support while the H-reflexes were examined. To examine H-reflex modulation, a PI H-reflex conditioning protocol was administered. The control H-reflex stimulus was set at 10% of M-max. PI was induced via conditioning of the soleus H-reflex with peroneal nerve stimulation at 100 msec prior to the test stimulus on the tibial nerve. The conditioning stimulus was delivered once the EMG activity of sternocleidomastoid muscle occurred as a trigger signal for lateral head movements. For the head up movement, an electrogoniometer was placed on the cervical spine and this was used as a trigger signal. Each mean of PI H-reflexes was normalized by the amplitude of control H-reflex (%). For data analyses, a 2 x 4 repeated measures ANOVA was used to determine each mean of H-reflex and PI-H reflex. RESULTS: The H/M-max ratio during standing without hands on the table was 41%. The ratio of PI H-reflex to the control H-reflex was 68.5% and 85.7% during standing with and without hands on the table, respectively. The mean ratio of PI H-reflex to control H-reflexes was decreased when the head was turned contralaterally by 8.4% and 13.5% with and without hands on the table, respectively. The ratio was decreased by 27.0% when the head was turned ipsilaterally with hands on the table, whereas it was not changed without hands on the table. CONCLUSIONS: The soleus H-reflex was modulated during head movements. PI to the soleus Ia afferents was decreased when the head was turned laterally, yet the modulation was dependant on the head movement and body stability.